有一人在平直马路上慢步（速度不变），他发现每隔t1分钟有一辆公共汽车迎面开过，每隔t2分钟有一辆公共汽车从身后开过．于是他计算出汽车站是每隔______分钟发一辆车．
设车的速度是a，人的速度是b，每隔t分钟发一辆车．
二辆车之间的距离是：at
车从前面来是相遇问题，人与车之间的距离也是：at
那么：at=t1（a+b）①
车从背后超过是一个追及问题，
那么：at=t2（a-b）②
则由①式和②式可知，t1（a+b）=t2（a-b），
化简得：b=
，a(t2-t1) t1+t2
所以，at=t1（a+b）=t1[a+
]，a( t2- t1) t1+t2
化简得：t=t1（1+
），t2-t1 t1+t1
从而可得出，t=
．2t1t2 t1+t2
故答案为：
．2t1t2 t1+t2
| 阅读理解。 | |
| Most people think of racing when they see greyhounds(灰狗) and believe they need lots of exercise. They can actually be quite lazy! Greyhounds are good at fast races but not long-distance run- ning. They do need regular exercise but they like to run for a short burst and then get back on the bed or a comfortable seat. Another misunderstanding is that greyhounds must be aggressive (好斗的) because they are big in size. In fact greyhounds love people and are gentle with children. Greyhounds can live for 12～14 years but usually only race for two or three years, and after that they make great pets. They don't need a lot of space,don't make a lot of noise,and don't eat a lot for their size. Normally, greyhounds can be as tall as 90cm. There is, however, a small-sized greyhound, which stands only 33cm. Greyhounds come in a variety of colors. Grey and yellowish-brown are the most common. Others include black, white, blue, red and brown or a mix of these. Greyhounds have smooth body coats ,low body fat and are very healthy. Because they're slim (苗条的)they don't have the leg problems like other dogs with the same height. But they do feel the cold ,especially since they would much rather be at home in bed than walking around outside. | |
| 1. The text is written mainly ________. | |
| A. to tell people how to raise greyhounds B. to let people know more about greyhounds C. to explain why greyhounds are aggressive D. to describe greyhounds of different colors | |
| 2. It can be inferred that greyhounds _______. | |
| A. love big dog houses B. like staying in bed all day C. make the best guard dogs D. need some exercise outdoors | |
| 3. Why does the author say that greyhounds.make great pets7 | |
| A. They are big in size. B. They live a very long life. C. They can run races for some time. D. They are quiet and easy to look after. | |
| 4. If you keep a pet greyhound,it is important _______. | |
| A. to keep it slim B. to keep it warm C. to take special care of its legs D. to take it to animal doctors regularly |